Electron beam tube and circuit



1 1960 G. M. w. BADGER ETAL 2,958,804

suac'mou BEAM TUBE AND cmcum 4 Sheets-Sheet 1 Filed May 19, 1958 vU w wE R x 1 3. Fl

BODY

POWER SUPPLY 7 LINE 62 VOLTAGE COLLECTOR POWER SUPPLY LOAD RESISTANCEINVENTOR.

esones M.w. BADGER OVER UNDER DONALD H. PREIST COUPLING invm AT TORNE Y4 Sheets-Sheet 2 I I I 20 I I 32 I 34 i 36 I I I I l 4" G. M. W. BADGERET AL ELECTRON BEAM TUBE AND CIRCUIT I l I I I I Nov. 1, 1960 Filed May19, 1958 LINE VOLTAGE F I I l l I I I l BODY SUPPLY INVENTOR. GEORGEM.W. BADGER DONALD H. PREIST BY M F "T ATTORNEY LINE VOLTAGE f COLLECTORSUPPLY 1960 G. M. w. BADGER ETAL 2,958,804

ELECTRON BEAM TUBE AND CIRCUIT 4 Shets-Sheet 3 Filed May 19, 1958 u h?m2: um

3 rlllFlllFlll FIIL INVENTOR. GEORGE MW BADGER DONALD H. PREIST git F ATTORNE Y NOV. 1, 1960 G w, BADGER A 2,958,804

ELECTRON BEAM TUBE AND CIRCUIT Flled May 19, 1958 4 Sheets-Sheet 4 us RFINPUT 8 7' I26 I20 I\ 2 I50 I38 I45 R.E OUTPUT LINE 1 v I44 VOLTAGE '43BODY COLLECTOR SUPPLY Fl g 7 lllll IX IIIII lllllllllll Ill Illll 1|||||llllllllllllllll I 1| l lllllllllrlll l 0EcREAs/-e LOAD RESISTANCEINCREASING- INVENTOR. GEORGE M.w BADGER J 5 DONALD H. PREIST ATTORNEYUnited States Patet ELECTRON BEAM TUBE AND CIRCUIT George M. W. Badger,Menlo Park, and Donald H.

Preist, Mill Valley, Califl, assignors to Eitel-McCullough, Inc., SanBruno, Calif a corporation of California Filed May 19, 1958, Ser. No.736,205

17 Claims. (Cl. SIS-5.39)

This invention relates to electron tubes and to methods of and means foroperating electron tubes. In particular, this invention relates tomethods of and means for protecting beam tubes, such as klystrons andtraveling wave tubes, from destruction due to improper operation.

In the operation of high power klystrons and traveling wave tubes one ofthe important problems is the self destruction of the tube due toimproper utilization of energy therefrom. In a klystron, for example, ifinsufficient power is coupled from the output cavity to the load (i.e.,if the klystron is improperly matched to its load, as by undercoupling)the output cavity will tend to become overheated, which may result indestructive thermal effects such as melting of seals or cracking ofinsulating envelope parts. Similarly, if a traveling wave tube isimproperly matched to its load the power reflected due to such mismatchwill tend to result in overheating of the helix and ultimate destructionof the tube.

Mismatch between a klystron and its load may occur for a number ofreasons. For example, when the circuit in which the klystron is to beoperated is being tuned to obtain optimum performance of the klystron,frequency variations will occur which will change the amount of couplingto the load. Similarly, during operation of either klystrons ortraveling wave tubes, the load on the tube may be wholly or partiallylost. For example, the output circuit into which the tube is operatingmay be shorted, or ice may form on an antenna to which the tube isconnected. Such loss of load will cause improper utilization of energyfrom the tube and may produce destructive overheating of the tube.

Therefore, it is an object of this invention to provide means forprotecting a klystron or a traveling wave tube from destruction due toimproper utilization of energy therefrom.

It is another object of this invention to provide a klystron or atraveling wave tube which is particularly designed to be used with acircuit which will protect it from destruction due to improperutilization of energy therefrom.

It is yet another object of this invention to provide a circuit capableof protecting a klystron or a traveling wave tube against destructiondue to improper utilization of energy therefrom.

Briefly, an electron tube according to one embodiment of this inventioncomprises an envelope containing an electron gun, a radio-frequencyinteraction means, and a collector electrode, the collector beingadapted to operate at a lower direct current potential with respect tothe cathode than that of the radio-frequency interaction means. Theelectron tube is operated in a circuit comprising a power supply meansproviding a first potential difference which is applied between thecathode and the collector of the tube and a second potential difference,greater than the first, which is applied between the cathode and theradio-frequency interaction means of the tube through a currentresponsive device. The current responsive device is adapted to interruptthe flow of beam current in the tube in response to a given current flowbetween the cathode and radio-frequency interaction means.

The invention possesses other objects and features of advantage, some ofwhich, with the foregoing, will be set forth in the followingdescription of our invention. It is to be understood that we do notlimit ourselves to this disclosure of species of our invention, as wemay adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure 1 is a simplified plan view in cross-section of a klystron andschematic representation of a circuit according to one embodiment ofthis invention;

Figure 2 is a graphical representation of certain electricalcharacteristics of a klystron in operation, showing the advantages ofthis invention over the prior art;

Figure 3 is a simplified plan view in cross-section of a klystron and aschematic representation of a circuit according to another embodiment ofthis invention;

Figure 4 is a simplified plan view in cross-section of a klystron and aschematic representation of a circuit according to still anotherembodiment of this invention;

Figure 5 is a simplified plan view in cross-section of a klystron and aschematic representation of a circuit according to a still furtherembodiment of this invention;

Figure 6 is a detailed plan view in cross-section of the type ofklystron shown in Figure 5;

Figure 7 is a simplified plan view in cross-section of a traveling wavetube and a schematic representation of a circuit according to thisinvention;

Figure 8 is a graphical representation of certain electricalcharacteristics of a traveling wave tube in operation showing theadvantages of this invention over the prior art.

Referring to Figure 1, a klystron 10 has been chosen for illustration inconnection with the basic principles of the subject invention since thegreatest amount of experimentation has been done with klystrons. Theklystron 10 comprises an electron gun 12 forming one end of an elongatedenvelope and a collector electrode 14 forming the other end of theenvelope. The intermediate portion of the envelope is formed by theradio-frequency interaction means or body 15 of the tube which comprisesa plurality of drift tube sections 16, 17, 18, 19 and 20 spaced fromeach other to form interaction gaps 22, 24, 26 and 28, and a pluralityof cavity resonator portions 30, 32, 34 and 36 bridging the interactiongaps 22, 24, 26 and 28.

The cavity resonator portions each comprise two metal end walls 38 and aceramic cylinder 40 sealed between the end walls 38. The ceramiccylinders 40 provide vacuum tight envelope walls and radio-frequencywindows through which radio-frequency oscillations may pass into theexterior non-evacuated portions of the resonant cavities represented bydotted lines 42. Such exterior portions of the resonant cavities areconductive, thus electrically interconnecting the drift tube sectionsand completing the R.F. interaction means or body 15 of the tube.

A ceramic cylinder 44 is sealed between an end wall 38 of the resonantcavity portion 36 adjacent the collector electrode 14 and a metallicflange 46 on the collector electrode 14, thus insulating the collectorelectrode from the remainder of the 'klystron 10.

The electron gun 12 of the klystron 10 comprises a cathode 48 and afocusing electrode 50. In order to produce a beam of electrons thefocusing electrode 50 is maintained at a zero or slightly negativepotential with respect to the cathode 48. This, along with the curvatureof the cathode 48, will focus the electrons from the cathode into a beamof slightly smaller diameter than the inner diameter of the drift tubesections. A positive potential applied to both the body 15 and thecollector .14 with respect to the cathode 48 will cause the beam to passaxially through the drift tube sections and into the collector. Aconventional magnetic system (not shown) is used to prevent the beamfrom spreading. All of the electrons in the beam will tend to have aconstant velocity which is determined by the potential applied to thebody and collector with respect to the cathode if no velocity modulationis applied to the tube and nearly all of the power in the beam will bedissipated at the collector in the form of heat.

As is well known in the klystron art, radio-frequency oscillations maybe introduced into the resonant cavity adjacent the electron gun 12 (asby means of an inductive loop 52, for example) which oscillations willproduce a varying electrostatic field across the interaction gap 22associated with such resonant cavity to modulate the velocity of theelectrons in the beam as they cross the interaction gap 22. Suchmodulation of the velocity of electrons in the beam results in bunchingof the electrons as they proceed along the drift tube. The bunches ofelectrons thus obtained cause varying electrostatic fields at each ofthe succeeding interaction gaps 24, 26 and 28 as they pass thereacross,thus inducing oscillations in the cavities 32, 34 and 36 associated withsuch gaps. Such oscillations will reinforce the electrostatic field ateach of the gaps which will further increase the bunching of theelectrons as they proceed along the drift tube. The oscillationsproduced in the cavity 36 adjacent the collector electrode 14 will be ofthe greatest intensity and may be coupled out of the cavity, by means ofanother induction loop 54, for example, as the power output of the tube.The electrons proceed on into the collector electrode 14 where theirresidual energy is dissipated in the form of heat.

Figure 2 is a graph of certain operational characteristics of aklystron. Since a klystron is designed to operate in resonant circuits,the load on a klystron will be essentially resistive. Thus, the baseline or abscissa of the graph of Figure 2 is divided into arbitraryunits of load resistance. With a klystron, as with any other powersource, optimum power in the load is obtained when the load resistanceis equal to the equivalent internal resistance thereof. Therefore apoint at the center of the base line or abscissa has been selected asthat value of resistance which equals the equivalent internal resistanceof the klystron and a vertical line has been drawn through such point.Values to the right of such line represent increasing load resistanceand values to the left of such line represent decreasing loadresistance. By definition and as shown by the curve labeled (W the powerdelivered to the load will be maximum at the value of load resistancerepresented by such line and will decrease for either increasing ordecreasing values of load resistance.

It the radio-frequency voltage in the output cavity of a klystron isplotted on the graph described above (as shown by the curve labeled 2 inFigure 2) it will be seen to decrease from some given value at optimumload resistance as the load resistance decreases and to increase fromsuch value as the load resistance increases. This is well known in theart and is as would be expected by analogy to other types of powersources.

In a klystron of the construction shown in Figure 1, the ceramiccylinder 40 in the output cavity 36 acts as a shunt impedance, acrossthe output circuit of the klystron. Thus, such ceramic cylinderintroduces power losses into the output circuit, such losses taking theform of heating of the ceramic cylinder. The heating of the ceramiccylinder is represented on the graph by the curve labeled T and is seento increase rapidly as the 19.89 I$ll l 9 increases. The rapid increasein heating of the ceramic cylinder is due to a number of effects. In thefirst place, the power loss in the ceramic cylinder will increase withthe square of the voltage thereacross. Secondly, the dielectric lossfactor of the ceramic cylinder tends to increase as its temperature goesup, the power loss or heating of the ceramic cylinder increasingaccordingly.

A further contribution to the heating of the ceramic cylinder is made bythe bombardment thereof by secondary electrons. Such secondary electronsare produced by the impingement of primary electrons from the beam onthe end or tip of the drift tube section 20 adjacent the output gap 28due to the beam spreading action of the output gap in extracting powerfrom the beam. The action of the output gap is such that the impingementof the primary electrons on the end of the drift tube section adjacentthereto will increase rapidly as the peak value of the radio-frequencyvoltage in the output cavity approaches the DC. voltage between cathodeand RF. interaction section.

Furthermore, the number and velocity of the secondary electrons producedby such impingement of primary electrons will increase in proportion tothe number of impinging primary electrons and therefore with the RP.voltage across the gap.

Thus, since the radio-frequency voltage in the output cavity increasesas the load resistance increases, the bombardment of the ceramiccylinder by secondary electrons and the heating of the ceramic cylinderdue to such bombardment will increase rapidly as the load resistanceincreases.

The variations in load resistance which are responsible for suchoverheating and destruction are largely due to coupling problems. A'variation in coupling has the effect of varying the resistive load onthe klystron and thus coupling may be substituted for load resistance onthe abscissas of the graph of Figure 2. The vertical line would thenrepresent both optimum load resistance and optimum coupling. Lesserdegrees of coupling would lie to the right of such vertical line andwould have the effect of increasing load resistance. Greater degrees ofcoupling would lie to the left of the vertical line and would have theeffect of decreasing load resistance.

It will be seen that if a klystron is undercoupled to its load theefiEective load resistance will increase, causing the output ceramic tooverheat and possibly resulting in destruction of the tube. For thisreason most klystrons are usually operated slightly over-coupled inorder to provide some degree of protection against destruction due tounder coupling.

However, coupling cannot always be accurately controlled. For example, aslight change in the orientation of a coupling loop can produce a largechange in coupling. In addition, variations in operating frequency, as,for example, when the klystron and circuit are being tuned to obtainoptimum power output, can produce very erratic variations in coupling.Furthermore, a broken output line or some similar physical occurrencecan cause a complete loss of load which is the ultimate of undercoupledconditions. Thus, a protective means is needed which will react to anundercoupling condition or loss of load to protect the tube fromdestruction.

In the operation of a klystron according to the prior art, there was nopractical means of protecting a beam tube from destruction due toundercoupling conditions. Referring to the graph of Figure 2, it will beseen that either undercoupling or overcoupling will produce the samevariation in power output. Furthermore, a comparatively small change inpower output due to undercoupling corresponds to a radical increase inceramic heating. Thus, monitoring of the power output of the tube willnot provide a reliable indication of the coupling condition thereof.Similarly, although the radio-frequency voltage in the output cavityincreases with underc upling, the IN? 0. such increase is so smallcompared to the radical increase in ceramic heating that monitoring ofsuch radio-frequency voltage is impractical as a means of providingprotection against destruction due to undercoupling.

However, according to this invention, undercoupling conditions may bequickly and accurately detected. This is accomplished by operating theklystron with the collector at a lower positive potential with respectto the cathode than the potential of the body with respect to thecathode and monitoring the current flow between the body and thecathode.

Referring again to the graph of Figure 2, the curve marked Ibdrepresents the current flow between the body of a klystron and thecathode thereof, when the collector and the body of the klystron are atthe same potential with respect to the cathode. Ideally, there should beno current flow between the body of the klystron and the cathodethereof, since all of the electrons from the cathode are supposed to befocused into a beam of slightly smaller diameter than the diameter ofthe drift tube sections. Thus, under ideal conditions, all of theelectrons emitted by the cathode would pass through the drift tubesections and impinge upon the collector. However, in actual practice, itis found that a certain small percentage of the electrons emitted by thecathode will not reach the collector but will impinge upon the body ofthe klystron. The impingement of electrons on the body of the tubeoccurs for a variety of reasons, among which are thermal effectsv andthe beam-spreading action of the various interaction gaps.

It will be seen that the body current Ibd described above increasesslightly when the tube is undercoupled. The increase in body current isdue to the increase in the radio-frequency voltage of the output gap 28when the tube is undercoupled. However, such increase in body current isso small in comparison to the radical increase in ceramic heating (Tthat it would be impractical to attempt to monitor the body current as ameans of providing protection against destruction due to undercoupling.

Thus, according to this invention, the collector is depressed, or inother words the tube is operated with the collector at a lower potentialwith respect to the cathode than the potential of the body with respectto the cathode. With the collector depressed, undercoupling causes thebody current to increase just as radically as the temperature of theceramic cylinder increases, as shown by the curve labeled Ibd The rapidincrease in body current with increasing load impedance (orundercoupling) when the collector is depressed is due to the increase inRF. voltage in the output cavity (curve e) which is also responsible forthe increased ceramic heating, as described above. For this reason theamount of body current is always representative of the ceramic heating.It will be seen that the increase in RF. voltage swing across the outputgap will have an increased effect on the electrons as they pass acrosssuch gap. Thus, certain of the electrons will be slowed down to such anextent that they will not possess sufiicient velocity to get into thecollector due to its low potential. Instead, they will be attracted backto the end of the body adjacent the collector due to its high positivepotential and will contribute to the body current. With specificreference to Figure 1, the slow electrons will not enter collector 14but will return to the drift tube section 20.

Referring again to Figure 1, there is shown a circuit according to thisinvention for protecting a klystron against damage due to undercoupling.A first power supply represented schematically at 56 is electricallyconnected between the cathode and the collector and provides a givenpotential difference therebetween (6 kv., for example). A second powersupply represented schematically at 58 (4 kv., for example) iselectrically connected between the collector and the ground, the

positive terminal of the supply being grounded. The RF. interactionmeans or body 15 of the tube comprising the interior cavity resonatorportions 30, 32, 34, 36 and the external portions represented by thedotted lines 42 are connected to ground through the coil 60 of a currentresponsive device 61. Therefore, it will be seen that the cathode andcollector are at negative potential with respect to ground, the cathodebeing more negative than the collector, while the body of the tube is atground potential. It should be understood that direct connections couldbe made without grounding any portion of the circuit or that the cathodeor the collector could be grounded instead of the body. The arrangementdescribed, with the body grounded, is preferred since it is believed tobe less dangerous to human life.

The switch contacts 62 associated with the coil of the currentresponsive device are placed in the input power lines to the powersupplies 56 and 58. The switch contacts 62 are normally closed, thusenergizing the power supplies 56 and 58. However, the current responsivemeans 61 is adjusted so that when a predetermined current fiows throughthe coil 60 the switch contacts 62 will be opened, thus disconnectingthe power supplies from the power source and inactivating the tube.Thus, since the collector is at a lower potential than the body of thetube, if the tube is undercoupled or the load impedance increases forany reason the body current will increase rapidly as described above.The resulting increased current flow through the coil of the currentresponsive device 61 will open the contacts 62 and inactivate the tubebefore overheating of the ceramic can occur. The action of the currentresponsive device 61 may be adjusted to be rapid or slow depending onthe needs of a particular tube or of the particular circuit in whichsuch tube is to be used. It should be understood that current responsivedevices other than that specifically described herein, such asbi-metal'lic resistance elements, for example, might be used, but thatregardless of the type of current responsive device used, it ispreferably one which is manually resettable.

The circuit described above also provides protection against othermalfunctioning such as misalignment of the beam or failure of themagnetic focusing equipment. Misalignment of the beam, for example, willresult in increased body current due to the increased impingement of thebeam on one or more of the drift tube sections. Such increased bodycurrent will flow through the coil 60 opening the contacts 62 andinactivating the tube.

However, the use of a lesser positive potential on the collector thanthe body of the tube, as described above, produces other problems. Forexample, the bombardment of the collector by the beam will producesecondary electrons. Such secondary electrons will tend to be attractedaway from the collector to the highly positive body. Some of suchsecondaries may be accelerated into the drift tube and will tend tocross the output interaction gap in the reverse direction and out ofphase with the electrons of the beam, thus interfering with theoperation of the tube.

The problem of secondaries may be satisfactorily overcome through theuse of a so-called flytrap collector. Such collector, as shown in Figure1, comprises a hollow member having a comparatively large volume withrespect to the size of the opening through which the beam passes. Thus,the secondary electrons which are produced by the impingement of thebeam on the interior of such member are shielded from the high positivefield of the body and are trapped within the collector. In other words,in a flytrap collector the angle which the secondary electrons see forescape will be greatly reduced since the cross-sectional area of thecollector is large in comparison to the area of the opening into thecollector.

The use of a depressed collector has an advantage in addition to that ofcausing the body current to increase rapidly with undercoupling. Thisadditional advantage is that the electron beam may be collected at alower potential. Thus, if the collector is depressed, a substantialpower savings may be realized. In a typical klystron of this kind bestresults have been obtained through the use of collector depression ofapproximately 40%. In other words, the potential of the collector withrespect to the cathode is 60% of the potential of the body with respectto the cathode. However, the collector may be depressed to a lesser orgreater extent within the scope of the subject invention.

Referring to Figure 3, another embodiment of this invention is shown.The tube according to the embodiment shown in Figure 3 is identical tothat shown in Figure 1 except that an auxiliary collector electrode 64is interposed between the end of the last drift tube section 20 and thecollector 14 and is insulated from both the drift tube section 20 andthe collector 14, and further that a probe 66 is placed in the collectorand insulated therefrom.

The probe 66 within the collector 14 is connected to the cathode 48through a resistor 67 so that it tends to assume the same potential asthe cathode. The purpose of the probe within the collector is to aid inthe suppression of secondary electrons within the collector.Furthermore, the probe has been found to be particularly desirable indepressed collector operation since it tends to remove any positive ionswhich may be trapped within the collector. The presence of such positiveions is undesirable in the collector since they will tend to perform afocusing action on the beam within the collector. Such focusing actionmay cause the beam to be concentrated on one spot on the interiorsurface of the collector, producing a hot spot, and may actually resultin melting of a small area on the collector wall releasing gases orperhaps even puncturing the collector. Therefore, the embodiment shownin Figure 6 represents a still further improvement on the flytrapcollector.

According to the embodiment shown in Figure 3, the cathode is connectedto ground through a high voltage power supply 68 (1O kv., for example)and to the collector through a lower voltage power supply 70 (6 kv., forexample), the negative terminal of each supply being connected to thecathode. As in the embodiment shown in Figure -l, the body of the tubecomprising the interconnected internal resonator portions 30, 32, 34, 36and the external resonator portions represented by dotted lines 42 areconnected to ground through the coil 60 of a current responsive device61, the switch contacts 62 being placed in the input power line to thepower supplies 68 and 70.

Similarly, the auxiliary collector is connected to ground through thecoil 72 of a second current responsive device 74. The contacts 76 ofsuch second current responsive device 74 are also placed in the inputlines to the power supplies 68 and 70. Under normal operating conditionsthe contacts 62 and 76 of both current responsive devices 61 and 74 areclosed. However, if excess current flows through either currentresponsive device 61 or 74 the contacts will be opened, disconnectingthe power supplies from the power source and inactivating the tube.

The embodiment of the invention shown in Figure 3 is specificallydesigned to take advantage of the fact that the rapid increase in bodycurrent due to undercoupling in depressed collector operation, as shownby the curve Ibd in Figure 2, as compared to the slight increase in bodycurrent under nondepressed collector conditions, as shown by the curvelbd is due almost entirely to the electrons of the beam which do notenter the collector but instead return to and impinge upon the end ofthe last drift tube section 20. By interposing the auxiliary collector64 between the collector 14 and the last drift tube section 20, suchelectrons are intercepted by the auxiliary collector 64. Thus, undernormal operating conditions substantially no current will flow throughthe current responsive device 74 connected to the auxiliary collector.However, when an undercoupled condition occurs, a very high current willimmediately result through the current responsive device 74 connected tothe auxiliary collector. Thus, it will be seen that the currentresponsive device 74 connected to the auxiliary collector need onlydiscriminate between substantially no current and a high current,whereas, referring to the embodiment of the invention shown in Figure 1,the current responsive device 61 must distinguish between a certainappreciable normal current flow and a higher current flow. Thus, theembodiment of the invention shown in Figure 3 enables a much finercontrol of the heating of the ceramic due to undercoupling.

The current responsive device 61 shown connected between the body of thetube and ground in Figure 3 will not respond in any way to undercouplingsince the auxiliary collector will receive all of the electrons whichtend to return to the body due to an undercoupled condition. Instead,the current responsive device 61 will provide protection againstmalfunctions of the tube such as misalignment of the beam or failure ofthe magnetic focusing system, as was mentioned with respect to theembodiment shown in Figure 1. Thus, all of the protective features ofthe embodiment of this invention shown in Figure l are provided by theembodiment shown in Figure 3 and in addition the response of theembodiment shown in Figure 3 to undercoupling is more discriminate.

Referring to Figure 4, still another embodiment of the subject inventionis shown. The klystron used according to the embodiment shown in Figure4 differs from that shown in Figure 3 in that it has a modulating anode78. Otherwise the tube is the same as was described with respect toFigure 3. The modulating anode 78 comprises an apertured electrodeadjacent the cathode which is insulated from both the electron gun 12and the body 15 of the tube 10. The aperture 80 of the modulating anodemay have a diameter approximately equal to the diameter of the drifttube sections and is of sufficient length to shield the cathode 48completely from the electric field of the body 15 of the tube. Thus, theemission of electrons from the cathode 48 is controlled entirely by thepotential difference between the cathode and the modulating anode 78 andis entirely independent of the potential difference between the cathodeand the body 15. If the modulating anode is at cathode potential (ornegative potential with respect to the cathode) no electrons will beemitted by the cathode and thus there will be no electron beam. If apositive potential is applied to the modulating anode with respect tothe cathode, electrons will be emitted by the cathode, formed into abeam by the focusing electrode 50, and accelerated into the drift tube.Due to the action of the focusing electrode, substantially no electronswill impinge upon the modulating anode 78 even though it is at a veryhigh positive potential with respect to the cathode 48. Since thequantity of electrons emitted by the cathode, or, in other words, thebeam current will increase as the positive potential on the modulatinganode increases and no electrons impinge upon such modulating anode, aneffective modulating means is provided which requires very low drivingpower.

The circuit according to the embodiment shown in Figure 4 is the same asthat of the embodiment shown in Figure 1 except that the switch contacts62 of the current responsive device 61 are not placed in the input powerline of the power supplies 56 and 58. Instead, the switch contacts 62are placed in the modulating anode circuit and function to apply acut-off voltage to the modulating anode 78 if undercoupling occurs. Inother words, under normal operating conditions the modulating anode 78is connected to ground through the contacts of the current responsivedevice. The modulating anode 78 is also connected to the cathode througha resistor 82.

Thus, the full voltage applied to the body of the klystron is developedacross the resistor 82 between the cathode and the modulating anode,placing the modulating anode at body potential with respect to thecathode, thus produe-ing full beam current. However, if an undercoupledcondition should occur, electrons returning to the body 15 of theelectron discharge device would pass through coil 60 of the currentresponsive device 61, tending to open the contacts 62. As soon as thecontacts are opened, the high voltage between modulating anode and thecathode will discharge through the resistor 82 and the modulating anodewill assume cathode potential, completely cutting off the electron beamand thus inactivating the klystron. As has been pointed out heretofore,the current responsive device may be of the manual- 1y resettable typeso that it may be reset and the tube returned to operation when thedifficulty has been corrected.

It will be seen that according to the embodiment shown in Figure 4 theinput to the power supplies is never interrupted even under undercoupledconditions. This arrangement may be particularly desirable inapplications where it is desired to place the klystron back intooperation as soon as possible after the undercoupled condition isrectified or where momentary undercoupling may be expected to occur.Since the power supplies are not turned off, the klystron will be readyfor operation as soon as the undercoupled condition has been correctedand the switch contacts 62 have been manually reset.

Referring to Figure 5, yet another embodiment of the subject inventionis shown. The klystron according to the embodiment shown in Figure is acombination of the klystron shown in Figure 3 with that shown in Figure4 in that it includes both an auxiliary collector 64 between the body 15and the collector 14 and a modulating anode 78 between the cathode 48and the body 15. According to this embodiment of the invention, as inthe embodiment shown in Figure 4, the modulating anode 78 is connectedto the cathode 48 through a resistor 82. In addition, a source 84 ofmodulating voltage is connected between the cathode 48 and modulatinganode 78 through the switch contacts 62 and 76 of two current responsivedevices 62 and 74. The coil 72 of one 74 of such current responsivedevices is connected between the auxiliary collector 64 and ground andthe coil 60 of the 61 other of such current responsive devices isconnected between the body 15 of the klystron and ground, as isdescribed in connection with the embodiment shown in Figure 3. Thus,under normal operating conditions the modulating anode 78 may beutilized to modulate the beam of the klystron as desired. However, if anundercoupled condition occurs, electrons will be returned to theauxiliary collector 64, causing a current fiow through the coil 72 ofcurrent responsive device 74 opening its switch contacts 76, and therebydisconnecting the modulator 84 and placing the modulating anode 78 atcathode potential which will cut off the beam. Similarly, if, due tosome malfunction of the tube or circuit, excess electrons from the beamimpinge upon the body 15 of the klystron, current will flow through thecoil 60 of the current responsive device 61 connected between the bodyand ground, opening its switch contacts 62 and placing the modulatinganode 78 at cathode potential to cut ofl? the beam. This embodiment ofthe invention combines all of the advantageous features of theembodiments heretofore described.

Referring to Figure 6, the structural details of an electron tubeaccording to the embodiment of Figure 5 are shown. The klystron shown inFigure 6 is specifically designed for high power operation. For example,the collector 14 of the klystron is provided with a cooling jacket 92having inlet 94 and outlet 96 connections whereby cooling fluid may becirculated about such collector. In addition, the auxiliary collector 64is adapted 10 to be liquid cooled by having a passageway 98 formedtherein with inlet and outlet connections 100 so that cooling fluid maybe circulated therethrough.

Among the other structural features of interest shown in Figure 6 arethe vacuum tight seals 102 between the ceramic cylinders 40 and themetal end walls 38. Each of such seals comprises a first metallicsealing ring 108 having one end sealed to an associated metal end wall38 as by brazing. A second sealing ring 110 is adapted to fit snuglywithin the first sealing ring 108 and has a flange which extendsinwardly across one end of an associated ceramic cylinder 40 and issealed to the end of the ceramic cylinder 40 as by brazing to a metalliccoating on the end of such cylinder 40. A ceramic backing ring 112 isbrazed to the opposite surface of the inwardly extending flange of thesecond sealing ring 110 from the ceramic cylinder 40 and is in slidingabutment with the metal end wall 38. The free ends of the sealing rings108 and 110 are sealed to each other as by brazing or welding tocomplete the vacuum tight joint. It will be seen that any stressescaused by the difference in radial expansion between a ceramic cylinder40 and the associated metal end wall 38 which may occur due to thermaleffects will be minimized by flexure of the sealing rings 108 and 110.The ceramic backing ring 112 provides for free sliding movement and atthe same time supports the full axial stress exerted on the seal 102.

Referring to Figure 7, an embodiment of this invention as applied to atraveling wave tube 114 is shown. As with the klystron heretoforedescribed, the traveling wave tube 114 comprises an electron gun 116 forproducing a beam of electrons, a body section 118 for radio frequencyinteraction with the beam, and a collector 120 for receiving theelectron beam after it has passed through the body 118. The electron gun116 comprises a cathode 122 which emits electrons and a focus electrode124 which forms such electrons into a beam. The collector 120 is of theflytrap design, similar to that shown in the klystron 10 of Figure 1, sothat collector depression, as described hereinabove, may be employed.

The body 118 of the traveling wave tube 114 is insulated from both theelectron gun 116 and the collector 1'20 by means of ceramic cylinders126, for example, and comprises a slow wave structure 128 supportedwithin a conductive shell 130. As shown in Figure 7, the slow wavestructure 128 is a helix arranged to surround the electron beam. Aradio-frequency wave is applied to the helix at the end thereof adjacentthe electron gun, as indicated at 132, and the helix is designed so thatthe wave is propagated along it at a velocity corresponding to thevelocity of the electron beam which passes axially through it. Thus, thewave interacts with the electron beam throughout the length of thehelix, tending to velocity modulate the electrons of the beam to producebunches of electrons in the beam, which bunches in turn tend toreinforce the wave. The reinforced beam is extraced from the helix atthe end thereof adjacent the collector, as shown at 134. The conductiveshell 130 serves to shield the helix and electron beam from theinfluence of any external electric fields and the helix is supportedwithin such shell 130 by insulating means 136 such as quartz rods spreadabout the outer periphery of the helix between the helix and the shell130.

It should be understood that the slow wave structure 128 may take avariety of forms other than the helix shown in Figure 7. For example,traveling wave tubes utilizing disc loaded wave guides, or interdigitedconductive structures, or filter networks as the slow wave structure 128are well known in the art. In addition, traveling wave tubes which makeuse of the backward wave which is propagated in a reverse directionalong the slow wave structure are known. It is believed that the subjectinvention is applicable to all of the traveling wave tube structuresmentioned above, including those which make use of the backward wave.According to the embodiment 11 shown in Figure 7, resistive elements 138are placed between the helix 128 and the shell 130 toward the gun end ofthe tube. Such resistive elements 138 serve to attenuate the backwardwave which is propagated along the helix and would tend to interferewith the desired operation of the tube by producing oscillations, etc.

The circuit according to the embodiment of the invention shown in Figure7 comprises high voltage (e.g., 10 kv.) power supply 140 connectedbetween the cathode 122 and ground and a lower voltage (e.g., 6 kv.)power supply 142 connected between the cathode 122 and the collector120. The shell 130 and the helix of the tube are connected to groundthrough the coil 143 of a current responsive device 144. Thus, it willbe seen that the body 118 of the tube is at ground potential, thecollector is at a negative potential with respect to the body, and thecathode is at a higher negative potential with respect to the body. Inother words, the circuit is designed for depressed collector operationof the tube. The switch contacts 145 of the current responsive device144 are placed in the input power line to the high voltage supplies 140and 142. Under normal operating conditions such switch contacts 145 areclosed, but if excess current flows through the coil 143 of the currentresponsive device 144 the contacts 145 will be opened, disconnecting thepower supplies and inactivating the tube.

Figure 8 is a graph similar to the graph of Figure 2, but showingcertain operational characteristics of traveling wave tubes. Since atraveling wave tube is inherently a much broader band device than aklystron, the power output (curve W of a traveling wave tube probablydoes not vary so radically with load resistance as does the power outputof a klystron. Nevertheless, there is an optimum load resistance, asrepresented by the vertical line in Figure 8, and if the load resistancedecreases to zero as with a shorted load, or increases to infinity, aswhen the load is lost due to the rupture of the output line, the poweroutput of the tube will decrease rapidly. Although the tube is notendangered by the shorting of the load to produce zero load resistance,it has been found that if the load resistance increases to high valuesapproximating infinity, as when an output line breaks, the helix will beintensely heated at the output end thereof, as is shown by the curve Th.Such heating of the helix is not fully understood but is believed to bedue to the increase in voltage which will occur at the output end of thehelix, as indicated by the curve e. Such increased voltage will causeincreased bombardment of the last few turns of the helix by electronsfrom the beam. Although the body current of a traveling wave tube undernon-depressed collector does not increase appreciably (as shown by thecurve lbd due to such bombardment, it is believed that secondaryelectrons are produced which may rebound between the last few turns ofthe helix, due to the rapidly changing electric fields present, to causethe excessive heating thereof.

However, by operating the tube with the collector depressed (i.e., withthe collector at a lower potential than the body) the body current maybe caused to increase rapidly as the load resistance approachesinfinity, as shown by the curve lbd The body current increases rapidlydue to the fact that certain of the electrons of the beam will not havesufficient velocity to enter the collector due to its reduced potential,but will return to the shell 130 and helix 128 of the tube, producing anincreased current flow through the coil 143 of the current responsivedevice 144, opening the contacts 145 thereof and inactivating the tube.

Thus, it will be seen that according to the subject invention novelcircuits are provided for the protection of beam tubes such as klystronsand traveling wave tubes during operation thereof. Furthermore, a novelstructure for beam tubes is provided, which construction enhances theprotective operation of the circuits. It should be understood that thecurrent responsive device,

described hereinabove as an electro-magnetic device, could take otherforms such as a bi-metallic element, for example, which will respond toexcess current flow. In addition, it should be realized that amodulating anode or an auxiliary collector or both, as described withrespect to Figures 3, 4 and 5, could be used in the traveling wave tubeembodiment shown in Figure 7. Also, changes can be made in the circuitrydescribed hereinabove which will not depart from the fundamentalteaching of the invention.

What is claimed is:

1. In combination an electron tube comprising an elongated envelope, anelectron gun including a cathode at one end of said envelope, acollector electrode at the other end of said envelope, and a radiofrequency interaction means interposed between said electron gun andsaid collector; and a circuit comprising a high voltage power supplymeans providing a given positive voltage and a higher positive voltage,means connecting said given positive voltage to said collector withrespect to said cathode, and other means connecting said higher positivevoltage to said radio frequency interaction means with respect to saidcathode, said other means including a current responsive device adaptedto interrupt the flow of beam current through said electron tube inresponse to a given current flow between said interaction means and saidcathode.

2. In combination an electron tube comprising an elongated envelope, anelectron gun including a cathode at one end of said envelope, acollector electrode at the other end of said envelope, and a radiofrequency interaction means interposed between said electron gun andsaid collector; and a circuit comprising a high voltage power supplymeans providing a given positive voltage and a higher positive voltage,and a current responsive device comprising an actuating means and anactuated means, said actuated means including a switch, said givenpositive voltage being directly connected to said collector electrodewith respect to said cathode, said higher positive voltage beingconnected to said radio frequency interaction means with respect to saidcathode through said actuating means of said current responsive device,said switch of said actuated means of said current responsive devicebeing interposed in the input power line to said power supply meanswhereby said current responsive device is adapted to interrupt theoperation of said power supply means in response to a given current flowbetween said radio frequency interaction means and said cathode.

3. An electron tube apparatus comprising an elongated envelope, anelectron gun including a cathode at one end of said envelope, acollector electrode at the other end of said envelope, a radio frequencyinteraction means in said envelope interposed between said electron gunand said collector electrode, high voltage power supply means connectedto said cathode, to said radio frequency interaction means and to saidcollector, said power supply means providing a given positive potentialto said collector with respect to said cathode and another positivepotential to said interaction means with respect to said cathode whichis higher than said given positive potential, an electro-magnetic switchcomprising a coil, a switch arm adapted to be actuated by said coil tomove away from a contact, said coil being electrically interposedbetween said radio frequency interaction means and said power supply,and said switch arm and contact being interposed in a power input lineto said power supply means, whereby a predetermined current flow throughsaid coil will move said switch arm away from said contact and open thepower input line to said power supply means.

4. In combination an electron tube comprising an elongated envelope, anelectron gun including a cathode at one end of said envelope, acollector electrode at the other end of said envelope, a radio frequencyinteraction means interposed between said electron gun and saidcollector, and an auxiliary collector electrode interposed between saidradio frequency interaction means and said collector electrode; and acircuit comprising a high voltage power supply means providing a givenpositive voltage and a higher positive voltage, means connecting saidgivenpositive voltage to said collector with respect to said cathode,other means connecting said higher positive voltage to said radiofrequency interaction means with respect to said cathode, and a currentresponsive device electrically connecting said higher positive voltageto said auxiliary collector electrode with respect to said cathode, saidcurrent responsive device being adapted to interrupt the flow of beamcurrent through said electron tube in response to a given current flowbetween said auxiliary collector electrode and said cathode.

5. In combination an electron tube comprising an elongated envelope, anelectron gun including a cathode at one end of said envelope, acollector electrode at the other end of said envelope, a radio frequencyinteraction means interposed between said electron gun and saidcollector electrode, and an auxiliary electrode interposed between saidradio frequency interaction means and said collector electrode; and acircuit comprising a high voltage power supply means providing a givenpositive voltage and a higher positive voltage, means connecting saidgiven positive voltage to said collector with respect to said cathode, afirst current responsive device connecting said higher positive voltageto said radio frequency interaction means with respect to said cathode,and a second current responsive device connecting said higher positivevoltage to said auxiliary electrode with respect to said cathode, eachof said current responsive devices being adapted to interrupt the flowof beam current through said electron tube in response to a givencurrent flow therethrough.

6. A combination as claimed in claim 4 wherein said current responsivedevices comprise an actuating means and an actuated means, said actuatedmeans including a switch, said higher positive potential being connectedthrough said actuating means, and said switch of said actuated meansbeing interposed in the input power line to said power supply means,whereby said current responsive devices are adapted to interrupt theoperation of said power supply means in response to a given current flowthrough the actuating means thereof.

7. In combination an electron tube and a circuit, said electron tubecomprising an elongated envelope, an electron gun at one end of saidenvelope including a cathode, a collector electrode at the other end ofsaid envelope, a radio frequency interaction means interposed betweensaid electron gun and said collector, and a beam modulating electrodeinterposed between said electron gun and said radio frequencyinteraction means, said circuit comprising a high voltage power supplymeans providing a given positive voltage and a higher positive voltage,a current responsive device comprising an actuating means and anactuated means, said actuated means including a switch, said givenpositive potential being directly connected to said collector electrodewith respect to said cathode, said higher positive voltage beingconnected to said radio frequency interaction means with respect to saidcathode through said actuating means of said current responsive device,said higher positive potential being also connected to said beammodulating electrode with respect to said cathode through said switch ofsaid actuated means of said current responsive device, and said beammodulating electrode being connected to said cathode through a resistor,whereby a given current flow through said actuating means of saidcurrent responsive device will open said switch of said actuated meansthereof reducing said beam modulating means to cathode voltage.

8. In combination an electron tube and circuit, said electron tubecomprising an elongated envelope, an electron gun including a cathode atone end of said envelope, a collector electrode at the other end of saidenvelope,

a radio frequency interaction means interposed between said electron gunand said collector, a beam modulating electrode interposed between saidelectron gun and said radio frequency interaction means, and anauxiliary collector electrode interposed between said radio frequencyinteraction means and said collector electrode; and a circuit comprisinga high voltage power supply means providing a given positive voltage anda higher positive voltage, a current responsive device comprising anactuating means and an actuated means, said actuated means including aswitch, said given positive voltage being directly connected to saidcollector electrode with respect to said cathode, said higher positivevoltage being connected to said radio frequency interaction means withrespect to said cathode, said higher positive potential being alsoconnected to the auxiliary collector electrode with respect to thecathode through the actuating means of the current responsive device andto the beam modulating electrode with respect to the cathode through theswitch of the actuated means of the current responsive device, and saidbeam modulating electrode being connected to said cathode through aresistor, whereby a given current flow through said actuating means ofsaid current responsive device will open said switch thereof reducingsaid beam modulating means to cathode voltage.

9. A combination as claimed in claim 8 wherein said circuit includes asecond current responsive device, said actuating means of said secondcurrent responsive device being interposed between said power supplymeans and said radio frequency interaction means, and said switch ofsaid actuated means of said second current responsive device isinterposed between said beam modulating electrode and said power supply.

10. A combination as claimed in claim 7 wherein said circuit includes ameans supplying modulating voltage connected between said cathode andsaid beam modulating electrode through said switches of said actuatedmeans of said current responsive devices.

11. An electron tube comprising an elongated envelope, an electron gunat one end of said envelope for generating a beam of electrons, acollector electrode at the other end of said envelope for receiving saidbeam of electrons, a radio frequency interaction means interposedbetween said gun and said collector electrode for interaction with saidbeam of electrons, and an auxiliary electrode interposed between saidradio frequency interaction means and said collector electrode, saidauxiliary electrode comprising an apertured plate through which saidbeam passes in its travel from the radio frequency interaction meanstoward said collector.

12. An electron tube comprising an elongated envelope, an electron gunat one end of said envelope for generating a beam of electrons, acollector electrode at the other end of said envelope for receiving saidbeam, said collector electrode comprising a hollow member having anenclosed volume and an opening through which said beam enters saidenclosed volume, a radio frequency interaction means interposed betweensaid gun and said collector for interaction with said beam, and anauxiliary collector electrode interposed between said radio frequencyinteraction means and said collector electrode, said auxiliary collectorelectrode comprising a metallic member having an aperture thereinthrough which said beam passes in its travel from the radio frequencyinteraction means toward said collector, electrons returning from saidcollector impinging upon said metallic member, said metallic memberhaving passageway formed therein through which a cooling fluid may becirculated.

13. A combination as claimed in claim 5 wherein said current responsivedevices comprise an actuating means and an actuated means, said actuatedmeans including a switch, said higher positive potential being connectedthrough said actuating means, and said switch of said actuated meansbeing interposed in the input power line to said power supply means,whereby said current responsive devices are adapted to interrupt theoperation of said power supply means in response to a given current flowthrough the actuating means thereof.

14. A combination as claimed in claim 8 wherein said circuit includes ameans supplying modulating voltage connected between said cathode andsaid beam modulating electrode through said switches of said actuatedmeans of said current responsive devices.

15. A combination as claimed in claim 9 wherein said circuit includes ameans supplying modulating voltage connected between said cathode andsaid beam modulating electrode through said switches of said actuatedmeans of said current responsive devices.

16. The combination according to claim 3, in which said collectorelectrode comprises a hollow member having an enclosed volumeconstituting a hollow interior and an opening through which the beamenters said hollow interior, the length of said hollow interior beingabout five times the diameter of said opening, whereby said collectorelectrode may be operated at a lower positive potential than said radiofrequency interaction means With respect to said cathode.

17. The combination according to claim 16, in which a metallic probe isinsulatingly mounted on the collector electrode opposite said openingand projects into the hollow interior of said collector electrode, saidcircuit including a resistor connecting said probe to the oathode.

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No.2,958,804

November 1, 1960 George M. W. Badger et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 15, line 15, for the claim reference numeral "3" read l (SEALAttestz ERNEST W. SWIDER Attesting Officer DAVID L, LADD Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION PatentNo, 2,958,804 November l 1960 George M. W. Badger et. al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 15, line 15,

for the claim reference numeral "3" read l Signed and sealed this 25thday of April 1961,

S AL his Attesting Officer Commissioner of Patents UNITED STATES PATENTOFFICE CERTIFICATION OF CORRECTION Patent No, 2,958,804 November 1 1960George Mrw. Badger et. air,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 15 line 15, for the claim reference numeral "3" read l Signed andsealed this 25th day of April 1961,

(SEAL Attestz- ERNEST W. SWIDER DAVID L; LADD Attesting OfficerCommissioner of Patents

